Mozilla has not only screwed up Firefox with their australis crap but have also essentially wiped out add-ons with their most current versions culminating (not immediately but, soon--perhaps as early as September 2018) in the removal of addons.mozilla.org.

"Up until now, Firefox, Thunderbird, and SeaMonkey users, used a single portal for extensions for their products. Even users of Firefox-based browsers could point their browsers to https://addons.mozilla.org/ to search for, download, and install browser extensions.Today's launch of https://addons.thunderbird.net/ changes that.

Thunderbird and SeaMonkey users cannot use Mozilla's Add-ons website anymore to download extensions for the email client or web browser as they need to use the new website instead. Redirects are in place to make the migration as smooth as possible for users. One core factor for the decision to fork the Thunderbird and SeaMonkey extension repositories was that Mozilla plans to remove legacy extensions from its platform in the near future (meaning 2018 likely).

Considering that Thunderbird and SeaMonkey do support legacy add-ons still, and will for the foreseeable future, it was clear that something needed to be done about it to provide users with options to download compatible extensions."